The microwave radiation emitted by a weather radar typically has a wavelength in the range between a few centimeters and 10 centimeters. Radiation with these wavelengths will reflect from objects that are sufficiently large, but will propagate right past smaller objects. Objects typically large enough to reflect microwaves emitted by a weather radar include:

• rain drops, snowflakes, hailstones, and other forms of precipitation;
• birds;
• insects;
• airplanes; and
• nearby buildings and mountains (if the radar is aimed too low).

A weather radar can typically see no farther than a couple of hundred miles, so the National Weather Service has established a network of radars spaced several hundred miles apart across the U.S. (see Figure 1) to reveal large-scale patterns of precipitation. The information gathered by the network of weather radars is sent to the National Centers for Environmental Prediction (NCEP) in Washington, D.C., which assembles the data into a composite radar summary (see example, see the accompanying weather radar map for the U.S.). (Sometimes these kinds of maps show areas outlined in parallelograms. These are areas where the National Weather Service has issued tornado "watches" or severe thunderstorm watches. A "watch" means that the NWS believes that conditions are favorable for the formation of tornadoes or severe thunderstorms in the very near future.)

In the 1990s the National Weather Service largely replaced its network of older "conventional" radars with about 135 WSR-88D Doppler radars. Like conventional radars, Doppler radars can detect the presence and intensity of precipitation, but unlike conventional radars, Doppler radars can also measure the speed with which precipitation moves toward or away from the radar, which provides indirect information about wind speeds and wind patterns in precipitating clouds. Such information can reveal the presence of strong wind shear near the ground (a hazard when aircraft try to take off or land), as well as circulation patterns (called mesocyclones) that can develop inside some severe thunderstorms before such storms produce tornadoes. (Note, however, that maps constructed from the NCEP composite radar data, such as the accompanying weather radar map, show only precipitation patterns, intensities, etc., not wind speeds and wind directions.)